Axon degeneration occurs frequently in many types of chronic neurodegenerative diseases and in injuries to axons caused by toxic, ischemic, or traumatic insults (Raff et al., 2002; Coleman and Perry, 2002). It may lead to separation of the neuron from its targets, resulting in loss of neuronal function. In the past, much effort has been focused on understanding the nature of neuronal cell death in these diseases (reviewed by Yankner and Yuan, 2001). However, strategies designed to prevent neuronal cell death have resulted in only limited success in preventing or delaying neurodegeneration (reviewed by Friedlander, 2003). One possibility is that neuronal cell death occurs too late in these diseases so that it may not be an efficient target for therapy. Thus, interfering with the process of axon degeneration may represent an additional and complementary therapeutic avenue for these diseases.
One model of axon degeneration is the self-destructive process observed at the distal portion of a transected axon upon injury, termed Wallerian degeneration (Waller, 1850). In vertebrates, the distal part of an axon can remain viable and conduct action potentials in vivo for up to a few days, after which it undergoes a rapid structural destruction where the axolemma and axonal cytoskeleton are dismantled (reviewed by Griffin et al., 1995; Gillingwater and Ribchester, 2001). Morphologically, such a degenerative process is characterized by a beading appearance followed by granular disintegration of the axons (reviewed by Griffin et al., 1995; Gillingwater and Ribchester, 2001). Axons undergoing Wallerian degeneration do not evidence detectable activation of the caspase family cysteine proteases (Finn et al., 2000), though calpain proteases appear to be activated (Glass et al., 2002).
Because most neuronal proteins are synthesized in the soma and carried to the axon by specialized axonal transport systems, degeneration of the transected axons has long been thought to result from starvation of necessary proteins and other materials. However, the recent discovery of a spontaneously occurring mutant mouse strain, C57BL/Wlds, whose axons survived for as long as weeks after transection (Lunn et al., 1989; Perry et al., 1990a; Glass et al., 1993), suggests that Wallerian degeneration involves an active and regulated auto-destruction program.
The Wlds phenotype has been attributed to the overexpression of a fusion protein (Mack et al., 2001) that consists of an intact nicotinamide mononucleotide adenylyltransferase (Nmnat) (Conforti et al., 2000), which functions in the synthetic pathway for adenine dinucleotide (NAD+), fused to 70 N-terminal amino acids of UFD2, an E4 protein involved in poly-ubiquitination (Koegl et al., 1999). The fusion gene is highly expressed in the Wlds nervous system, and induces a Wlds phenotype when expressed in wild-type mice. It is not known how it interferes with Wallerian degeneration (Mack et al., 2002): the N-terminal portion is but a tiny fragment of the native 1173 amino acid UFD2, and this fragment is absent in the functionally conserved homologue of UFD2 in yeast (Koegl et al., 1999), so functional significance of this fragment is not apparent. And although the fusion protein retains Nmnat activity, there are conflicting unpublished data on deletion mutant function (Coleman et al., 2002; Raff et al., 2002; He et al., 2003). In any event, the fusion protein is primarily nuclear, so it presumably acts indirectly to protect axons. (Mack et al., 2001).
The ubiquitin proteasome system (UPS) has been implicated as a common mechanism for selective protein degradation in a variety of biological processes, such as axonal pathfinding (Campbell and Holt, 2001, 2003) and synapse formation (for example, DiAntonio et al., 2001). UPS disruption has been associated with numerous pathologies, particularly neuropathies. For example, an intragenic deletion in the gene encoding ubiquitin carboxy-terminal hydrolase causes the axon degeneration phenotype of the gracile axonal dystrophy (gad) mouse (Saigoh et al., 1999). Furthermore, the UPS is involved in neurodegenerative diseases such as Alzheimer's, Parkinson's and poly-glutamine repeat diseases, wherein a common feature is the abnormal accumulation of proteins in plaques (Miller and Wilson, 2003). Inhibiting the UPS may enhance the pathogenic accumulation of proteins (Bence et al, 2001), and several neurodegenerative diseases appear to be caused by genetic mutations that inhibit the UPS (Miller and Wilson, 2003).
Nevertheless, proteasome inhibition has been a proposed therapeutic strategy for treating cancer, reperfusion injury, and inflammatory diseases like asthma, rheumatoid arthritis, multiple sclerosis and psoriasis (Elliot et al., 2003, J Mol Med 81, 235–45; Pye et al., 2003, Am J Physiol Heart Circ Physiol 284, H919–26; Elliot et al., 2001, Am J Clin Pathol 116, 637–46; Phillips et al., 2000, Stroke 31, 1686–93). Here we report that pharmacological inhibition of the UPS can delay axon degeneration associated with a variety of neuropathies, particularly acute neuropathies, apparently by stabilizing microtubules.